Still have this issue in 552p4

In my case, I need to add the addComponent line inside OnEnable function to work:

using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class FixToggleGroup : MonoBehaviour {

public ToggleGroup toggle_group;
private Toggle toggle;

void Start () {
toggle = gameObject.GetComponent<Toggle>();
}
void OnEnable() {
toggle = gameObject.GetComponent<Toggle>();
toggle.group = toggle_group;
}
}

Thanks for the code snippet

// this script is helpful to work around the bug in the meantime
using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class FixToggleGroup : MonoBehaviour {

public ToggleGroup toggle_group;
private Toggle toggle;

void Start () {
toggle = gameObject.GetComponent<Toggle>();
}
void OnEnable() {
toggle.group = toggle_group;
}
}

In which version of 4.6 is this fixed?

I can reproduce in b21.